Self-supporting cable

ABSTRACT

Self-supporting cables include at least one insulated conductor that includes a conductor having at least one wire and an insulation around the cable conductor. The cable further includes at least one longitudinally extending shield band and a jacket. The shield band is rigid in a radial direction and includes undulations that extend mainly in a tangential direction. The shield band includes undulations which correspond to the jacket undulations. A weak radially acting compressive force causes the jacket undulations and the shield band undulations to cam into each other, such that the force of gravity acting on the cable between the cable fixing points is transmitted into the conductors, and an axially acting force in the absence of slippage between the different cable layers. The cable becomes self-supporting by virtue of the mechanical strength of the conductors.

FIELD OF INVENTION

The present invention relates to self-supporting cables.

BACKGROUND OF THE INVENTION

As will be evident from FI 33129 and EP 0 461 794, for instance, it isknown to make aerial cables self-supporting by integrating a supportline in the cable. It is also known to provide cables of improvedtensile strength by embedding tension force relieving members in thecable insulation, c.f. U.S. Pat. No. 4,956,523. It is also known toprovide a cable of high tensile strength, by placing a reinforcementcomprising, e.g., glass fibre wires immediately inwards of the outerjacket; c.f. DE 17 90 251 or EP 0 268 286.

SE 8105835-6 teaches a cable that includes a shield band about eachinsulated conductor of the cable. The cable is not self-supporting,however.

SUMMARY OF THE INVENTION

One problem with known self-supporting cables is that they consist ofmany different insulated conductors or many different layers. This makesthe cable expensive and complicated to manufacture, and in some casesdifficult to install.

One object of the present invention is to provide a self-supportingcable that can withstand the strain caused by a falling tree, forinstance.

Another object of the present invention is to provide a self-supportingcable of simple and inexpensive manufacture and which can be easilyinstalled.

These objects are achieved in accordance with the invention with a cablethat comprises at least one insulated conductor where each insulatedconductor includes a conductor that has a conductor insulation. Alongitudinally extending shield band provided with grooves orcorresponding undulations is applied around each insulated conductor,either completely or partially. The cable includes an outer extrudedjacket. As the jacket is extruded, corresponding undulations are alsoformed in the jacket and in the conductor insulation. The undulations onthe various cable conductors grip into one another when the cable issubjected to mechanical load, so as to prevent sliding or slippagebetween the various conductors. This enables the load generated by theweight of the cable to be transferred inwardly to the cable conductorsas an axially directed force that the conductors carry by virtue of itsinherent mechanical strength among other things.

The inventive self-supporting cable has the advantages of being simpleand inexpensive in manufacture and of being easily installed. Otheradvantages are that the cable need not be made round and that the shieldbands form a mechanical protection that is particularly effectiveagainst punctiform pressures.

The invention will now be described in more detail with reference topreferred exemplifying embodiments thereof and a also with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one cable embodiment.

FIG. 2 is a cross-sectional view of one cable embodiment, taken on thelines A—A in FIG. 3.

FIG. 3 is a longitudinal sectional view of one cable embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Cable

FIG. 1 is a perspective view of a cable, while FIG. 2 is across-sectional view of the same cable, from which it will be seen thatthe cable includes three insulated conductors 1, 2, 3. The number ofconductors may be more or fewer than three. Each conductor 1, 2, 3includes a conductor 4 and a conductor insulation 5.

The conductor 4 is comprised of a plurality of drawn, combined andtwisted wires 11, comprised of aluminium or copper, for instance. Theillustrated embodiment includes nineteen wires. Although it is possibleto use only one wire 11, mechanical strength will be enhanced by using aplurality of wires. Swell yarn or swell powder may be incorporated inconjunction with combining the wires, as protection against the ingressof water. An innermost semi-conductor layer 12 is extruded around aconductor 4. An insulating layer 13 is extruded around the innermostsemiconductor layer 12, and an outer semiconductor layer 14 is extrudedaround said insulating layer 13. The two semiconductor layers 12, 14 maybe comprised of an electrically conductive plastic and the insulatinglayer 13 may be comprised of cross-linked polyethene (PEX). The threelayers 12, 13, 14 make up the conductor insulation 5.

The cable conductors 1, 2, 3 are twisted, or twined, so as to enhancetheir mechanical strength. Each insulated conductor 1, 2, 3 is partiallyembraced by a shield band 6. Poorer mechanical strength can be expectedwhen only one insulated conductor 1 is used and the shield band 6should, in this case, fully embrace the conductor 1.

Although there will preferably be used one shield band 6 with eachconductor 1, it is conceivable to use more or fewer shield bands 6 thanthe number of conductors 1 present.

The shield band 6 includes undulations 22, 23 such as grooves or thelike that extend essentially tangentially and that are comprised, forinstance, of a fabric of tin-plated copper wires. Alternatively, groovedmetal foil or undulating copper wires between plastic foils may be used.

A jacket 7 is extruded around all conductors 1, 2, 3. The jacket 7 mayconveniently be comprised of a strong polyethene or some other materialwith low cold-flow, so as to avoid deformation of the jacket in thepassage of time. The material will also preferably have a certain degreeof elasticity that will provide flexibility, see below.

The shield band 6 is sufficiently rigid in its radial direction toenable the undulations 22 thereon to be reproduced on the inner surfaceof the jacket 7, these undulations being referenced 21; see FIG. 3.Grooves 24 are also preferably formed on the outer semiconductor layer14, and hence this layer must be relatively soft. The outersemiconductor layer 14, however, must be sufficiently strong to beprevent it from being easily broken, and it may also be strippable.These criteria are satisfied when the outer semiconductor layer 14includes an inner relatively hard layer and an outer softer layer.

The shield bands 6 will also preferably be soft in an axial direction,so as to result in a flexible cable and so that the outermostsemiconductor layers 14 will not be crushed when the cable bends or issubjected to load.

On the one hand the undulations 21 on the jacket 7 and the undulations22 and on the other hand the undulations 23 on the shield bands and theundulations 24 on the outer semiconductor layers firmly grip in oneanother when the cable is subjected to load. This prevents undesiredslippage or creepage between the different cable conductors, therewithenabling the jacket 7 to be extruded around the conductors more looselythan would otherwise have been necessary. The resultant cable is thusmore flexible than it would have been in the absence of saidundulations. This is because the jacket 7 is able to slide against theshield bands 6 to some extent, in the absence of load on the cable. Thissliding of the jacket 7 is made possible because the undulations 21 onthe jacket 7, which is slightly elastic, “jump” in the undulations 22 onthe shield bands 6. Corresponding “jumps” can also occur between theshield band undulations 23 and the undulations 24 on the outersemiconductor layers. This is desirable, because undesirable tension andcompression forces would otherwise occur as the cable is bent. Becausethe undulations 21, 22, 23, 24 are in mutual engagement after the cablehas been bent, the extent to which the cable “springs back” when thebending force is relieved will be reduced.

The self-supporting capacity of the cable is achieved by virtue of themutual engagement of on the one hand the jacket undulations 21 and theshield band undulations 22, and on the other hand the shield bandundulations 23 and the undulations 24 on the outer semiconductor layers,when a weak radially acting compressive force is applied on cable fixingor installation points. This enables the gravitational force acting onthe cable between the cable fixing or installation points as an axiallyacting force to be transmitted into the conductors 4 in the absence ofsliding or slippage between the different cable layers, wherewith thecable becomes self-supporting by virtue of the inherent mechanicalstrength of the conductors 4.

The aforedescribed use of shield bands 6 obviates the need for fillingin order to maintain the integrity of the shield construction. Theaforedescribed use of shield bands 6 also enables the cable to be givenfor example a triangular cross-sectional shape, as shown in FIG. 1,instead of needing to be round. When desiring a more watertight cable,the empty spaces 15 may be filled with swell yarn or swell powder.

Cable Manufacture

In one method of manufacture, an electro-refined aluminium rod is firstdrawn to a wire of suitable diameter or thickness, preferably 2-3 mm. Aplurality of wires 11, preferably 19 in number, are then broughttogether and twisted or twined to form a conductor 4, optionally withthe inclusion of swell yarn 16 or swell powder.

The conductor 4 is then fed into an extruder in which three insulationlayers 12, 13, 14 are extruded simultaneously on the conductor 4. Thethus produced cable conductor 1 is then cooled with water and thereafterwound onto a drum.

Three cable conductors 1, 2, 3 are then delivered to a cabling machinein which each of said conductors is provided with a respective shieldband 6, whereafter the cable assembly is twisted about its longitudinalaxis. The shield bands 6 are held in position by locking said bandssecurely at regular intervals with the aid of a thread or wire 31,preferably a non-spun thread, or a strip 31 of some suitable material.The strip 31 will preferably be made of a material similar to the jacketmaterial, so that the strip is able to fuse into the jacket as thejacket is extruded thereon. Alternatively, metal strips or the like maybe used.

The twisted or twined cable conductors 1, 2, 3 are then fed to anotherextruder, in which a jacket 7 is extruded at a pressure with which theshield band undulations 22 will be reproduced on the inner side of thejacket 7 in the form of undulations 21. It is also preferred to formundulations 24 on the outer semiconductor layer 14 at this stage ofmanufacture. The tightness with which the jacket is extruded on thecable conductors is a question of balance. If the jacket is extruded tootightly, the cable will become very rigid and “jumping” of theundulations 21, 22 over one another becomes difficult, as will beevident from the aforegoing.

The manufactured cable is then cooled and wound onto a drum.

What is claimed is:
 1. A self-supporting cable comprising at least oneinsulated conductor that includes a conductor having at least one wireand a conductor-insulation, at least one longitudinally extending shieldband, and a jacket surrounding the at least one insulated conductorwherein each shield band is provided with undulations that extendgenerally tangentially, and is radially rigid; and the jacket hasundulations that correspond to the shield band undulations, the jacketbeing slidable relative to the shield band, wherein said jacketundulations and said shield band undulations grip into one another inresponse to relatively low radially acting pressure forces on cablefixing points thereby preventing the relative sliding between the jacketand shield band, such that tension forces and gravitational forcesacting on the cable between said fixing points can be transmitted intothe conductor as an axially extending force in the absence of slippage,such that the cable becomes self-supporting by virtue of intrinsicmechanical strength possessed by the conductor.
 2. A self-supportingcable according to claim 1, wherein the insulation on said at least oneconductor comprises an inner semiconductor layer, an insulating layer,and an outer semiconductor layer, wherein the inner and outersemiconductor layers are formed of an electrically conductive plastic;and the outer semiconductor layer includes undulations, the undulationson the outer semiconductor layer grip with the shield band undulationsin response to pressure that acts radially on the cable.
 3. Aself-supporting cable according to claim 2, wherein the outersemiconductor layer includes an inner relatively hard layer and an outerlayer that is softer than said inner layer.
 4. A self-supporting cableaccording to claim 2, wherein the shield band has low rigidity in itsaxial direction, such as to provide a flexible cable.
 5. Aself-supporting cable according to claim 1, wherein the at least oneshield band is comprised of a woven metal wire fabric of tin-platedcopper wires.
 6. A self-supporting cable according to claim 1, whereinsaid at least one shield band includes undulating metal wires disposedbetween plastic foils.
 7. A self-supporting cable according to claim 1,wherein said at least one shield band includes undulating metal foil. 8.A self-supporting cable according to claim 1, wherein the jacketundulations grip in shield band undulations; and the elasticity of thejacket is such as to enable the jacket undulations to jump within theshield band undulations as the cable bends.
 9. The self-supporting cableof claim 1, wherein the jacket undulations are on an inner surface ofthe jacket.
 10. A cable comprising: a conductor; a conductor-insulation;a longitudinally extending shield band, the shield band comprisesundulations on both an inner surface and an outer surface of the shieldband; a semiconductor layer between the shield band and theconductor-insulation, the semiconductor layer comprises undulationswhich grip into the undulations on the inner surface of the shield band;and a jacket, the jacket comprises undulations which are normally spacedfrom the shield band undulations but which grip into the undulations onthe outer surface of the shield band upon application of radially actingpressure forces to the cable.
 11. The cable of claim 10, wherein thejacket undulations are slightly elastic and can jump in the undulationson the outer surface of the shield band.
 12. The cable of claim 11,wherein the semiconductor layer undulations can jump in the undulationson the inner surface of the shield band.